Electrolytic preparation of alkali metal ferricyanides



July 18, 1944 H. R. NEUMARK 2,353,781

ELECTROLYTIC PREPARATION OF ALKALI-METAL FERRICYANDES Filed May 2, 1942 l ATTORN EY Patentecl July 18, 1944 UNITED STATES PATENT OFFICE ELECTROLYTIC PREPARATION oF LKALIv METAL FERRICYANIDES Hans vR. Neumark, Forest Hills, N. Y., assigner to General Chemical Company, New York, N. Y., a corporation ot New York Y Application May 2, 1942, Serial No. 441,499

8 Claims. (Cl. 204-91) i On completion of electrolysis, finished anolyte solution-predominantly potassium ferricyanide contaminated with potassium ferrocyanide and potassium hydroxideis withdrawn from the cell, cooled to crystallize potassium ferricyanide, and iiltered to recover potassium ferricyanide crystals. The mother liquor nitrate resultingfrom such crystallization contains varying quantities of potassium ferricyanide and ferrocyanide, and substantial amounts of potassium hydroxide. This mother liquor is then used in somey way or other in the make-up of another potassium ferrocyanide solution to .be supplied to the electrolytic cell for a succeeding operation.

From the above equation it will be noted that, as electrolytic oxidation proceeds, for every mol of potassium ferricyanide formed one mol of KOH is also produced. In practice, approximately half of this KOH is retained in the cathode chamber while the balance of such KOH works thru the porous walls of the diaphragm into the anolyte solution which, when finishedy is withdrawn from the cell. Hence, the mother liquor Yresulting from crystallzing potassium ferricyanide out of an anolyte solution' contains a substantial quantity of potassium hydroxide which was produced dur-V ing electrolysis. Mother liquor of high potassium hydroxide concentration cannot be used in making up a subsequent ferrocyanide starting solution without rst getting rid of some of the potassium hydroxide in order to prevent a detrimental 'build-up of this compound in the liquors in the system. As far as I am aware, all prior proposals, directed to reduction of potassium hydroxide concentration of potassium ferricyanide liquors, involve treatment of such liquors with potassium hydroxide neutralizing agents which, because of their compositions and characteristics,

result in introduction of foreign compounds intol Presence of these Airnthe circulating liquors. purities gives rise to difficulties which can be oiiset only` more or less by frequent and cumbersome recrystallization of the potassium ferricyanide crystals in order to obtain a product of reasonable purity. Hence, reduction of potassium hydroxide concentration of ferricyanide solutions has been a problem in the art.

One of the principal objects of this invention is provision of a processfor lowering the alkalil metal hydroxide concentration of alkali metal ferricyanide liquors without introducing undesirable impurities into such liquors and without requiring losses of alkali metal or cyanide values.

I have found that the objects of the invention may be accomplished by treating alkali metal ferricyanide-ferrocyanide-hydroxide liquors with certain agents which function to convert the alkali metal of the excess or undesired alkali metal hydroxide to alkali metal iron-cyanide compounds which are soluble and remain in solution in the treated liquor, and to convert the hydroxide component of the excess alkali' metal hydroxide to an iron hydroxide precipitate which may be removed from the treated solution by simple illtration.

Thus, I am enabled to neutralize or lower the alkali metal hydroxide content of liquors to any extent desired, to suit the particular operation at hand, and transform the alkali metal constituent of the alkali metal hydroxide so neutralized to a soluble alkali metal iron-cyanide and remove from the system the hydroxide radical of the alkali metal hydroxide so neutralized, all without introducing into the operation any material which adversely aiects the'purity of the ultimate products, and without causing loss of alkali metal or cyanide values as by liquor bleed-oil.

Practice of the invention comprises treating a liquor, of the type described and containing an undesired amount of alkali metal hydroxide with a treating agent reactable with alkali metal hydroxide to form soluble alkali metal iron-cyanide and to precipitate iron hydroxide. More particularly I have found that suitable treating agents are iron salts, of an acid of the group consistingV of ferro and ferri cyanic acids, such saltsbeing reactable with the alkali metal hydroxide contained in the liquors treated to form soluble alkali metal iron-cyanide and to precipitate iron hydroxide. Examples of suitable neutralizing or treating agents are Prussian blue, understood to be ferrie ferro cyanide, Fe4(Fe(CN) s) 3; Turnbull's blue, understood to be ferrous ferricyanide,

and a more or less white iron salt, understood to be ferrous ferrocyanide, FezFe CNl s.

In carrying out the invention, in apparatus such as illustrated diagrammatically on the accompanying flow sheet and for example as applied to manufacture of potassium ferricyanide, when starting operation a potassium ferrocyanide solution may be formed in make-up tank l0 by dissolving in water K4Fe(CN)e.3H2O crystals in quantity to form a substantially saturated sowithout use of mother liquor from a previous op.

eration may comprise from 150 to 250 g. p. l. of K4Fe(CN) and from 2 to 10 g. p. l. of KOH. It is noted that when forming a potassium ferrocyanide starting solution as in tank Il, whether such solution is made up from K4Fe(CN)s.3HzO crystals 'and .extraneous potassium hydroxide or from K4Fe(CN)e.3HzO crystals and potassium rerricyanide-ferrocyanide-hydroxide mother liqf 4 uor fromV a previous operation, such starting solution is made up so as to have what may be referred toas a predetermined KOH concentration which may vary for instance from 2 to 25 g.p`.` l., depending uponthe nature of the particular operation.

The startingz solution is run into 'the anolyte chamber oi' a cell ,provided preferably with a nickel anode and with a steel cathode surrounded by a porous ceramic diaphragm Il forming a cathode chamber Il. Circulation oi solution in the anolyte chamber,.and a solution temperature ofthe order of 4050 C. may be maintained by lany convenient means. Ordinarily, during theprogress of oxidation, temperature should not exceed 50 C. in vorder to prevent decomposition ofA -ierricyanide to iron hydroxide, and temperature may be kept as high as say 40 C. to avoid crystallization of potassium ferricyanide. The cathode chamber is iilled with a weak solution of KOHwhich functions primarily as a conductor between the cathode and the walls of the surrounding porous diaphragm. Whenfthe current Vis turned on, reaction proceeds in accordance with the above Equation (1) and potassium ferrlcyanide is formed. As previously noted, during thecourse of reaction irl the cell one mol of KOH is formed li'or every mol of potassium ferricyanide produced, about half of the KOH thus formed remaining in the cathode chamber, the balance of such KOH working its way thru Athe porous walls of the diaphragm and into the anolyte solution. The improvements constituting the present invention are primarily directed toward removali'rom the system, at a latter stage, of preferably'that portion of potassium hydroxide which is vcontained in the finished anolyte and which was produced during oxida-V tion.

` On completion of oxidation to the extent desired, the anolyte solution at temperature of, e. g., 4050 C. is filtered in filter Il toseparate any solids, and the iiltrate is run into a cooler and crystallizer Il. A typical filtered anolyte solution may contain from 350 to 400 g. D. l. of IQFe(CN)c, from 20 to 37g. p. l. of K4Fe(CN)a, and from to 32 g. p. l. of KOH. The anolyte solution is cooled to room temperature-and agitated for 6-8` hours to effect crystallization of KaFe(CN) s product whichis filtered out in iilter I8 and suitably dried.

As a result of this iiltration operation, there is produced in line a potassium ferricyanide mother liquor containing some potassium ferrocyanide and also some KOH the total of which is made un of (a) the KOH fed into the anolyte chamber with the incoming potassium ferrocyanide starting solution plus (b) that'portion of the KOH which was produced during oxidation and which is present in the finished anolyte solution as withdrawn from the cell. A representative mother liquor filtrate may comprise 260 to 300 g. p. l. of KsFe(CN) s, 20 to 37 g. p. l. of K4Fe(CN) e, and 17 to 33 g. p.1. of KOH.

In accordance with the preferred embodiment of the invention, such a mother liquor is treated in tank 22 with one of the previously mentioned agents to neutralize a desired amount of KOH and lower the KOH content of the mother liquor to such a degree that the liquor may be used in forming another potassium terrocyanide makeup solution for a succeeding operation of the electrolytic cell. The preferred extent of KOH content reduction of a givenbody of mother liquor will be understood from-consideration of the ,operating expedient that in commercial proc- 'tothe potassium ferrocyanide solution make-up apparatus-III, contains an amount of KOH such that a succeeding make-up solution (formed by mother liquor) has the operators desired predetermined KOH concentration. Viewed from another angle, extent of KOH neutralization of a total body of mother liquor is such as to neutralize a quantity of KOH which approximately corresponds with that portion of potassium hydroxide which was contained in the finished anolyte and which was produced during oxidation. In other words, the preferred embodiment of the invention Vas to extent of KOH neutralization gets rid of the normal KOH build-up taking place during one complete passage of liquor thru the system.

I find that the most satisfactory treating or neutralizing agent is Prussian blue,

which when addedto 'a potassium ferricyanide mother liquor containing potassium hydroxide reacts with potassium hydroxide in accordance with the following:

Thus, theV potassium of the'neutralized potassium hydroxide is converted back to soluble potassium ferrocyanide which is the essential constituent of a starting solution fed into an electrolytic cell,

and the hydroxide content of the neutralized potassium hydroxide is converted to ferrie hydroxide which may be filtered out of the partially vanide crystal crop from a liquor subsequently solution to be fed into the electrolytic cell. Hence, the amount of KOH to be neutralized maybefound by liquor analysis, and the theoretical quantity of treating agent needed may be readily calculated. In operation, it is preferred to use an excess of about to 10% by weight of treating agent over the calculated theoretical requirement. For example, in the case of Prussian blue, rbest results are obtained when using a'bout 1.4-1.5 parts by weight for every part of KOH to be neutralized.

I further flnd that to facilitate the neutralizing reaction in tank 22 and obtain a satisfactorily illterable iron hydroxide precipitate, the treating operation should be carried out at temperatures from about 35 C. to not more than 50 C. Since thev mother liquor from the potassium ferricy anide filter I8 is usually at room temperature, it

is preferred to heat up the mother liquor, prior to treatment with the neutralizing agent, to within the temperature range indicated. The neutralizing agent is then added and the mass is agitated for from say 10 to 2O minutes during which time the reaction indicated by equation (2) takes place and proceeds to completion. Thereafter the liquor is iiltered to remove the iron hydroxide, and the partially neutralized iiltrate is returned to thepotassium ferrocyanide make-up solution apparatus. practice of the process a mother liquor containing about 273 g. p. l. of K3Fe(CN)s, about 26.2 g. p. l. of K4Fe(CN)6 and 29.2' g. p. l. of KOH was agitated for minutes at a temperature of 40 C. with 1.45 parts fby Weight of Prussian blue per part of KOH to .'be neutralized. The liquor was illtered and the clear filtrate contained 22.6 g. p. l. 1

oi' KOH.

In practicing the invention, as applied for example to neutralization of mother liquor, it is not necessary to subject an entire body of such liquor to actual contact with neutralizing agent. To illustrate, a half portion of a given body of mother liquor may be treatedv with the full quantity of neutralizing agent needed to effect the desired extent of neutralization of the total body of mother liquor. On completion of neutralization, and following ltration of the treated half portion of mother liquor, the resulting iiltrate and the untreated portion of the mother liquor are combined, and as is apparent the KOH concentration of the composite is the same as would have been the case had the `entire body of mother liquor been treated. This procedure has the advantage, in some operations, of avoiding heating up and iiltering the entire volume of mother liquor.

Thus far the KOH neutralizing operation has been described in connection with treatment of mother liquor resulting from ltering out of potassium ferricyanide crystals. The potassium hydroxide concentration reduction feature of the invention may be applied similarly to liquor at any convenient point in the system. For example, the needed quantity of treating agent may be added to In one example oi the potassium ferricyanide anolyte solution as at fed into the potassium Ierricyanide cooling and crystallizing apparatus. Accordingly, reduction of KOH content of this point inthe liquor circuit .is not as desirable as the previously discussed mother liquor treatment. I

When Turnbulls blue or ferrous Ierrocyanide are used as neutralizing agents, the reactions-taking place are much the same as previously indicated and apparently proceed respectivelyas follows However, it is preferred to use rPrussian blue asv it has been found .that the resulting iron lhydroxide precipitate is more readily iilterable. The next overall most satisfactory treating agent is Turnbulls blue.

The principles of the invention are also applicable to` neutralization of the sodium'hydroxide content of sodium ferricyanide liquors produced by electrolytic oxidation of sodium ferrocyanide. In this operation, a sodium ferrocyanide-solution containingy sodium hydroxide Vis electrolytically oxidized tor produce a sodium ferricyanide anolyte containing some unoxidized sodium ferrocyanide and the NaOH initially fed into the anolyte chamber plus the NaOH formed during oxidation and present in the anolyte solution as withdrawn from the anolyte chamber, such oxidation of sodium ierrocyanide being generally.

comparable with the previously described oxidation of potassium ierrocyanide. case of the sodium ferricyanide anolyte solution, often it is desirable to completely remove the sodium hydroxide, and hence in accordance with the invention the quantity of ktreating agent employed may be that required to completely neutralize the NaOH content of the anolyte liquor and form an iron hydroxide precipitate and a.

corresponding quantity of soluble sodium ferrocyanide. After treatment of the solution with a treating agent, e. g., preferably Prussian blue attemperatures of 35-50 C. and with agitation, the anolyte solution is filtered to separate out precipitated iron hydroxide and form a sodium ferricyanide filtrate containing more or less sodium ferrocyanide.

droxide, treating alkali metal ferricyanide liquor containing alkali metal hydroxide with an iron salt, of an acid of the group `consisting of ferroand fri-cyanic acids, reactable with alkali metal hydroxide to form soluble alkali metal ironcyanide and to precipitate iron hydroxidethereby reducing the alkali metal hydroxide content of said liquor and converting alkali metal constituent oi' alkali metal hydroxide to soluble alkali metal iron-cyanide, separating iron hydroxide from said liquor, and returning soluble alkali metal iron-cyanide constituent of the resultinfr separated liquor to the tion.

However, in the subsequent starting solu- 1 2. The process for making alkali metal fer-rif "cyanide'comprising forming a starting solution `containing'-'falkali metal -ierrocyanida electrolyt- ;fiically:oxidizingA said alkali metal ferrocyanide solution under conditions fammi alkali ymetalv 'rierricyanide liquor containing alkali metal hydroxide, treating alkali metal i'erricyanide liquor containing alkali metal hydroxide with Prussian :blue reactable" withalkali metal 'hydroxide to form-soluble alkali` metal vferrocyanide' and to --f-precipitate" iron hydroxide, thereby reducing the alkali metal hydroxide content of said liquor and converting alkali metal' .constituent of alkali met- K, ,al-hygroxideto soluble alkali metal Ierrocyanide, 'separating iron hydroxide from said liquor, and

returning solublealkali metal fe'rrocyanide con- *stituentf-'of the-resulting separated liquor to the subsequent startingsolution The ,process for making; potassium Vferrilcyanide comprising :forming-a starting solution aQntaniI-lf; potassium ierrocyanide, electrolytically oxidiz'i. g said potassium ferrocyanide solution under conditions to form-.potassium'ferri- -cyanide fliquor 4containing potassium. hydroxide, recovering solid' potassium ferricyanide from said Qsplution,I treating result-ing potassium ferricyanidezmother .liquorcontaining potassium hy- .droxide with, an iron salt,` of an acid of the group .droxide content Yof said mother liquor and con- 'verting potassium constituent oi potassium hydroxide,-'to soluble potassium iron-cyanide, sepa- -ratingirori hydroxide from said mother liquor, .andreturningthe residualA liquor containing solub lepotassiunrv-iron-cyanide kto the subsequent 14, The process formaking potassium i'erricy-V anidecompriing forming astarting solution containingpotassium ferrocyanide, electrolytically oxidizing saidr potassium ferrocyanide lsolution under conditions to yform. a potassiumV ferricyanide anolyte liquor. containing potassium hydroxide -produced `.during said oxidation, withdrawing ,finished anolyte4 solution from the oxidizing. one, recoyering solid potassium ierricyanide fromfsaid anolytesolution, treating resulting potassium Aferricyanide mother liquor containing potassium hydroxide .produced during said oxidation'with an iron salt, of an acid of 'the group consisting voiferro; and ferri-cyanic acids, reactablewithpotassiumhydroxide to form soluble potassium iron-cyanide and -to precipitate -iron hydroxide, thereby reducing the potassium` hydroxide content of said mother liquor and converting potassiumlconstituent of potassium hydroxide to, soluble-potassium iron-cyanide, the

quantity-o1' ysaidsalt'being suchxas to effect re-` duction of the-potassium hydroxide content of the. total other liquor by an 4amount approximately corresponding to that portion of potassium hydroxide which was contained in said iinished .anolyte and which was produced guring saidoxidation, separating ironY hydroxide from said mother liquor, and returning the residual liquor containing soluble potassium iron-cyanide to thevsubsequent starting solution.

5.The process for making potassium ferricyanide comprising iorminga Astarting solution containing potassium ferrocyanide, electrolytically `oxidizing 'said potassium ferrocyanide solution underl conditions ,to form a potassium ferricyanide anolyte liquor Acontaining potassium hydroxide produced during said oxidation, withdrawing iinished anolyte solution from the oxidizing zone', re-

covering solid potassium ferricyanidefrom said anolyte solution, agitating result in'gpotassium hydroxide, thereby reducing the potassium hydroxide'content oi said mother liquor and con- L verting potassium constituent of potassiumv hydroxide to soluble potassium ferrocyanide, the

quantity of Prussian blue being ysuch as'to eiiect reduction of the potassium hydroxide contento! the total mother liquor by an amount approximately corresponding to thatportion of potas- 1 sium-hydroxide which was contained in said iinvas to eifect reduction of the potassium Y ished anolyte and which was Aproduced during said oxidation, separatingiro'n hydroxide Lfrom said mother liquor, and returning the residual liquor containing soluble potassium ferrocyanide to the A subsequent starting solution.

6. The processior making potassium ferricyanide comprising forming anginitial potassium ferrocyanide starting solution having a predetermined concentration .of potassium hydroxide. electrolytically oxidizing said potassium ferrocyanide solution under conditions to form a potassium ierricyanide anolyte liquor containing potassium hydroxide produced during said oxidation, withdrawing finished anolyte solution from the oxidizing zone, recovering solid potassium ierricyanide fromV said anolyte solution, treating resulting potassium ferricyanide mother liquor containing potassium hydroxide with an iron salt, of an acid of the group consisting of ferroierri-cyanic acids, reactable with potassium hydroxide to form soluble potassium iron-cyanide and to precipitate iron hydroxide, thereby reducing the potassium hydroxide content of said mother liquor and converting potassium constituent of potassium hydroxide to soluble potassium iron-cyanide, the quantity of said salt being such hydroxide content of the total mother liquor by an amount such that when said total mother liquor is used to form a subsequent starting solution, the potassium hydroxide concentration of such subsequent starting solution approximately corresponds with the potassium hydroxide concentration of said initial starting solution, separating iron hydroxide from said mother liquor, and returning the residual liquor` containing soluble potassium ironcyanide to theV subsequent starting solution.

7. The process for making alkalimetal ferricyanide comprising formingV a starting ysolution containing alkali metal ferrocyanide, electrolytically oxidizing said alkali metalferrocyanide solution under conditions to form alkali metal i'erricyanide liquor containing alkali metal hydroxide, treating `alkali metal ferricyanide liquor containing alkali metal hydroxide with material, of the group consisting of Prussian blue and ferrous ferrocyanide, reactable withl alkali metal hydroxide to yform soluble alkali metal ferro.- cyanide and to precipitate iron hydroxide, thereby reducing the alkali metal hydroxide content of said liquor and converting alkali metal constituent of alkali metal hydroxide to soluble alkali metal ierrocyanide, separating iron hydroxide froml said liquor, and returning soluble alkali metalferrocyanide constituent of the resulting f separated liquor to the subsequent starting soluandv 8. The process for making potassium ferricyanide comprising forming a starting solution containing potassium ferrocyanide, electrolyticaily oxidizing said potassium ferrocyanide solution under conditions to form potassium ferricyanide liquor containing potassium hydroxide, recovering solid potassium Ierricyanide from said solution, treating resulting potassium ferricyanide mother liquor containing potassium hydroxide with Prussian blue i-eactable with potassium hydroxide to form soluble potassium ferrocyanide and to precipitate iron hydroxide, thereby reducing the potassium hydroxide content of said mother liquor and converting potassium constituent oi' potassium hydroxide to soluble potassium ferrocyanide, separating iron hydroxide from said mother liquor, and returning the residual liquor containing soluble potassium ferrocyanide to the subsequent starting solution.

HANS R. NEUMARK. 

